The present invention relates to a nondispersive infrared gas analyzer.
The U.S. Letters Pat. No. 3,925,667 describes a gas analyzer having two infrared beams whose intensity is periodically modulated (interrupted) by a revolving chopper, and the two modulated beams each pass through similar cells containing the sample gas to be analyzed. One of the beams emerging from the respective cell is passed to a single absorption detector chamber, the other beam passes serially through two absorption chambers. All these absorption chambers pertain to a pneumatic detector and the end-absorption chambers are connected to opposite sides of another chamber which is biparted by a membrane whose deflection on account of pressure differences is electrically monitored. The third absorption chamber connects to the same pressure side on the membrane its serial companion is connected to, but through a flow impedence to offset long term pressure differentials between them.
The infrared gas analyzer of that patent operates on the basis of what can be described the principle of negative filtering. This principle is of advantage over so-called positive filtering, if the sample gas contains a component which has an absorption band that overlaps the band of the component to be detected. The cross-sensitivity is simply more pronounced in positive filtering which produces errors accordingly. These aspects have been more fully described generally by Werner Schaefer in "Gasanalyse mit dem Uras bei kompliziert zusammengesetzten Messgasen", from Chemie-Ing.-Techn., 33. Year, 1961/No. 6.